The caloric value of fuel oils can be converted to mechanical energy or electric energy by means of combustion, and is generally used in boilers, turbines or engines. Many countries have set standards and regulations to control the maximum permissible discharge level of air pollutants to protect the environment. Thus, reducing the emitted concentration and amount of air pollutants, such as nitrogen oxides (NOx) and carbon oxides (COx), total hydrocarbons (THC), particulate matters (PM) and smoke as a result of fossil fuel combustion, is an important subject in environmental protection today.
To overcome the problems described above, some methods have been developed to form emulsion fuel oils by mixing fuel oil, water and preferable a specific additive.
Emulsification of water and light combustible oils improves the combustion efficiency in the combustion process which resulting in reduced harmful emission of air pollutants and reduced specific oil consumption, without creating undesirable side effects such as secondary emissions or hazardous waste.
In order to improve the lubrication effect of the emulsion and to strengthen the bindings between oil and water for the creation of emulsions with high stability and long storage ability, an emulsifying additive may be added in the emulsification process.
However, the emulsion fuel oils made from light oils and water formed with processes known from the prior arts has disadvantages of instability after long time storage. The resulting emulsion fuel oil is not very stable, and the water and oil are readily separated from each other. Most of the emulsions of light fuel oils can only maintain its emulsifying stability for a few days at room temperature. Thus, the emulsion fuel oil is usually used immediately as it is formed, and is unfavorable for large-scale production and long-distance transportation.
Further, it is sometimes preferable to prepare an emulsion without the addition of an emulsifying additive, and these emulsions are even more unstable, often less than 1 hour.
U.S. Pat. No. 4,394,131 describes a stable combustion fuel emulsion of a petroleum fuel having a small percentage of water dispersed therein as droplets of a size of about 0.5 micron. The experimental section of this patent discloses that stable (more than three month) emulsions have been obtained for residual oils with a viscosity of 400 SSU (example 1) and 4000 SSU (example 2). The most stable emulsions in example 1 and 2 have Water Droplets Size of 0.5 micron, and these emulsions are formed by a rotary impact mill operating at a speed of 1650 rpm.
In contrast, the object of the present invention is to provide stable emulsions of light oils, i.e. oils with a viscosity below 2 mm2/s at 40° C. The methods and the emulsions disclosed in U.S. Pat. No. 4,394,131 are not capable of providing stable emulsions of light oil and water.
The inventors of the present invention have surprisingly found that the stability of water in fuel oil emulsions of light oils, without or with added emulsifying additive, can be improved even further by reducing the particle size distribution and the mean size of the particles dramatically, i.e. to a mean size of about 0.2 micron (200 nm). Therefore, the present invention provides improved emulsion of light fuel oils, without emulsifying additive, to overcome the instability problems described above.
Further, the inventors have also found that the stability of water in light fuel oil emulsions with an emulsifying additive can be improved by reducing the particle size distribution and the mean size of the particles. Therefore, the present invention provides an improved emulsion light fuel oil with emulsifying additive, with an increased stability.